communications failing due to large packetsize?

Dear LWIP users,

I am using the lwip 1.1.1 on two embedded applications. They talk to
eachother and read/write to FIFO's. The pbufs are chopped up to 2 bytes on
the sending-end. When I send 1350 bytes everything goes well, but 30 bytes
more and it fails.  The debug (and ethereal) both say 'packet discarded due
to failing checksum' and then my connection is broken. The receiver-side
isn't acking the failed package, so my senders' queue is only growing. I'm
new in the TCP/IP world so I'm a bit clueless about where to look.

I use these defines:

#define MEM_SIZE                                2*1024*1024
#define TCP_MSS                         1460
#define TCP_SND_BUF                             2*1024*1024
#define TCP_SND_QUEUELEN                        2*1024*1024
#define MEMP_NUM_TCP_SEG                        2*1024
#define MEMP_NUM_PBUF                           16
#define TCP_WND                                 4*1024
#define WND_OFFSET                              633


thank you for your time,

Frans